The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone bits and to an enhanced cutting structure for such bits. Still more particularly, the invention relates to the placement of gage cutter elements on the rolling cone cutters at locations that increase bit durability and rate of penetration.
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to drill a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or xe2x80x9cgagexe2x80x9d of the drill bit. As used herein, xe2x80x9cbit diameterxe2x80x9d and xe2x80x9cgage diameterxe2x80x9d refer to the same parameter.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide on the bottom of the borehole as the bit is rotated, with the cutters engaging and disintegrating the formation material in the path of the bit. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. Each cone includes a plurality of cutter elements in its outer conical surface. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material. The chips are carried upward and out of the borehole by a drilling fluid, which is pumped downwardly through the drill pipe and out of the bit, and recirculates to the surface via the annulus between the drill pipe and the borehole wall.
The earth disintegrating action of the rolling cone cutters is enhanced by the cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as xe2x80x9cTCIxe2x80x9d bits, while those having teeth formed from the cone material are known as xe2x80x9csteel tooth bits.xe2x80x9d In each case, the cutter elements on the rotating cutters functionally breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a xe2x80x9ctripxe2x80x9d of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits that will drill faster and longer and that are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (xe2x80x9cROPxe2x80x9d), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements upon the cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.
Bit durability is, in part, measured by a bit""s ability to xe2x80x9chold gage,xe2x80x9d meaning its ability to maintain a full gage borehole diameter over the entire length of the borehole. Gage-holding ability is particularly vital in directional drilling applications, which have become increasingly important. If gage is not maintained relatively constant, it becomes more difficult, and thus more costly, to insert drilling apparatus into the borehole than if the borehole had a constant diameter. For example, when a new, unworn bit is inserted into an undergage borehole, the new bit will be required to ream the undergage hole as it progresses toward the bottom of the borehole. Thus, by the time it reaches the bottom, the bit may have experienced a substantial amount of wear that it would not have experienced had the prior bit been able to maintain fall gage. This unnecessary wear will shorten the bit life of the newly-inserted bit, thus prematurely requiring the time consuming and expensive process of removing the drill string, replacing the worn bit, and reinstalling another new bit downhole.
To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, decreased ROP, increased loading on the other cutter elements on the bit, and may accelerate wear of the cutter bearing and ultimately lead to bit failure.
In addition to the heel row inserts, conventional bits typically include a gage row of cutter elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. In this orientation, the gage cutter elements generally are required to cut both the borehole bottom and sidewall. The lower surface of the gage row insert engages the borehole bottom while the radially outermost surface scrapes the sidewall of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in rows that are disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.
In general, the cutting action of the cutter elements at the borehole bottom is typically a crushing or gouging action, while the cutting action at the sidewall is a scraping or reaming action. Because differing forces are applied to the cutter elements by the sidewall than the borehole bottom, it is desired to separate these cutting duties so that the corresponding cutter elements can be optimized.
One U.S. Patent that teaches the separation of sidewall and bottom cutting duties is U.S. Pat. No. 5,372,210. U.S. Pat. No. 5,372,210 teaches the benefits of distributing the inserts on each rolling cone such that a more rounded borehole corner is formed. The ""210 patent provides a xe2x80x9ctransition rowxe2x80x9d of cutter elements, which drill the rounded corner between the vertical sidewall and the borehole bottom. The purpose of this configuration is to reduce concentrated side forces and to facilitate directional drilling while minimizing gage wear. The ""210 patent further provides an arrangement whereby the gage row insert diameter on each rolling cone is different than the others. Additionally, the ""210 patent stresses the importance of providing each rolling cone with a gage row that acts upon the same portion of the borehole sidewall, redundantly. According to the ""210 patent, a bit having a cone with a gage row that is placed to cut a portion of the borehole sidewall closer to or farther from the borehole bottom than the gage rows on the other cone(s) will experience a force imbalance that can cause bit gyration, bit whirl, or off-center rotation.
Another patent, U.S. Pat. No. 3,452,831, teaches a rolling cone for use in a bit, wherein the rolling cone supports multiple circumferential rows of cutter inserts that all serve to ream to the borehole sidewall to the same diameter. The ""831 patent teaches that the reaming or heel row cutters are redundant with the corner cutting inserts. Various other configurations for accomplishing the desired drilling efficiency, ROP and bit life are disclosed in the art.
Nevertheless, it has been discovered that, in an effort to maintain a full gage borehole, the conventional (prior art) arrangement of cutting elements on a bit provides an overpopulation of cutter elements responsible for cutting the borehole corner. This overpopulation of cutter elements, though effective for maintaining a full gage borehole, corresponds to inadequate forces acting on the borehole bottom portion of the corner as well as ineffective usage of the cutter elements in the inner rows, thus resulting in reduced bottom hole cutting efficiency. Hence, there remains a need for a bit that is optimized for certain types of formations where powerful bottom hole cutting ability is crucial.
Accordingly, there remains a need in the art for a drill bit and cutting structure that are more durable than those conventionally known and that will yield greater ROP""s and increase the footage drilled while maintaining a full gage borehole. Preferably, the bit and cutting structure provide these advantages without requiring the compromises in cutter element toughness, wear resistance or hardness that are common in conventional bits.
The present invention provides an earth boring bit for drilling a borehole of a predetermined gage. In many formation types, the bit provides increased durability, ROP and footage drilled at full gage as compared with similar bits of conventional technology. The bit includes a bit body and one or more rolling cone cutters rotatably mounted on the bit body. Each rolling cone cutter includes a generally conical surface, an adjacent heel surface, and preferably a circumferential shoulder therebetween. A row of heel cutter elements are secured to the cone cutter and have cutting surfaces aligned with the gage curve, as is known in the art. At least one row of fanned-gage cutter elements are secured to the cone cutter and have cutting surfaces that cut to full gage. The cutter elements in the fanned-gage rows are fanned out or strategically positioned at various locations along the borehole sidewall such that the amount of borehole bottom cutting responsibility of each fanned gage row is progressively reduced as its sidewall contact point is positioned farther from the hole bottom. The bit further includes a plurality of inner row cutter elements that are secured to the cone cutter on the conical surface. The placement of the gage rows in the fanned configuration serves to transfer a greater amount of cutting force to the adjacent inner rows, thereby increasing the cutting efficiency of the inner rows.
According to the invention, the cutter elements may be hard metal inserts having cutting portions attached to generally cylindrical base portions that are mounted in the cone cutter, or may comprise steel teeth that are milled, cast, or otherwise integrally formed from the cone material. The orientation, mounting angle, diameter, extension and shape of cutter elements in the fanned-gage rows may be the same for all the cone cutters on the bit, or may vary between the cone cutters in order to achieve a desired balance of durability and wear characteristics for the cone cutters. The fanned-gage row cutter elements may be mounted along or near the circumferential shoulder, either on the heel surface or on the adjacent conical surface.
Where the fanned-gage cutter elements and first inner row cutter elements are inserts, the ratio of the diameter of at least one of the fanned-gage row inserts to the diameter of the inner row inserts is preferably not greater than 0.75 for certain preferred embodiments of the invention.
In another embodiment, on the cone containing the first inner row, the extensions of the fanned-gage row cutter element and the first inner row cutter element define a step distance, where the ratio of the step distance to the extension of the first inner row cutter element is greater than 0.5.
In another preferred embodiment, the fanned-gage row cutter element of each cone defines an oversize angle, wherein the difference between the oversize angles of any two cones is at least 0.5 degrees.
In another preferred embodiment, a bit in accordance with the invention comprises a bit body having a bit axis, at least two and more preferably at least three rolling cone cutters rotatably mounted on said bit body and having a generally conical surface and an adjacent heel surface, a plurality of inner row cutter elements positioned on at least one cone cutter in a first inner row, a plurality of gage-cutting cutter elements positioned in a first fanned-gage row on a first one of said one cutters, a plurality of gage-cutting cutter elements preferably positioned in a second fanned-gage row on a second one of said one cutters, and a plurality of gage-cutting cutter elements positioned in a third fanned-gage row on a third one of said one cutters, wherein the cutter elements in said first fanned-gage row, said second fanned-gage row, and said third fanned-gage row contact the gage curve at different points.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned to contact the gage curve at least two different points, each cutter element has an extension, and the extensions of the uppermost fanned-gage row cutter element and the first inner row cutter element define a step distance, and wherein the ratio of the step distance to the extension of the first inner row cutter element is greater than 0.5.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned to contact the gage curve at least two different points, each cutter element has an extension, and the fanned-gage rows are positioned such that the cone cutters have different oversize angles.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned to contact the gage curve at least two different points, each cutter element has an extension, and the ratio of the diameter of the largest fanned-gage row cutter element to the diameter of the first inner row cutter elements is preferably not greater than 0.75.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned to contact the gage curve at least two different points, each cutter element has an extension, and the lowermost fanned-gage row contains more cutter elements than any other fanned-gage row.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned on lands on cone cutters and contact a gage curve at least two different points.
In another embodiment, the bit further includes a plurality of heel-cutting cutter elements positioned on at least two cone cutters on lands.
In another embodiment, the first, second, and third fanned-gage rows of gage-cutting cutter elements are positioned to contact a gage curve at different points and the calculated scraping distances are different with the same given cone to bit speed ratio for said gage-cutter cutting elements.
In another embodiment, the first, second, and third fanned-gage rows of gage-cutting cutter elements are positioned to contact a gage curve at different points, wherein the lower-most fanned-gage row is positioned on the same cone cutter as the third inner row cutting elements.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned to contact a gage curve at different points, wherein the first and second inner rows extend further than the fanned-gage rows.
In another embodiment, the first and second fanned-gage rows of gage-cutting cutter elements are positioned to contact a gage curve at different points, wherein the vertical force exerted on the first and second fanned-gage rows during drilling is different. More specifically, the vertical force on the upper fanned-gage row(s) is much less than the vertical force on the lower fanned-gage row(s).
The invention permits the borehole sidewall and corner cutting load to be distributed between the cutter elements in the fanned-gage rows such that the lowermost fanned-gage cutter elements cut the majority of the bottom portion of the borehole corner. Consequently, the fanned-gage cutter elements that are positioned farther from the borehole bottom perform progressively less borehole bottom cutting and see less vertical load from the hole bottom, while performing increased sidewall reaming duties. This configuration also allows cutter elements in the first inner row to be paired on the same cone cutter with the uppermost fanned-gage row, with the result that the first inner row cutter elements can penetrate deeper and more aggressively into the formation. This positioning also allows the first inner row cutter elements to be moved closer to the borehole sidewall, thus further protecting the fanned-gage rows from bottom hole cutting responsibilities, and enables each of the rows of cutter elements to be optimized in terms of materials, shape, and orientation so as to enhance ROP, bit durability and footage drilled at full gage.